Method for producing circular dna formed from single-molecule dna

ABSTRACT

There is provided a method for producing a circular DNA which consists of a circular DNA formed from a single-molecule DNA and which does not comprise circular DNA formed from multiple-molecule DNA. According to the method of the present invention, a circular DNA molecule formed only from a single-molecule DNA can be reliably produced.

TECHNICAL FIELD

The present invention relates to a method for producing circular DNAthat is formed only from single-molecule DNA, a novel adapter used inthe above-mentioned method, and a kit for producing circular DNA,comprising the novel adapter.

The present invention further relates to a method for identifying and/ordetecting a gene using single-molecule circular DNA produced by theabove-mentioned method. In particular, the invention relates to a methodfor identifying and/or detecting a fusion gene that causes variouspathologic conditions.

BACKGROUND ART

Vector method is a conventional gene analysis method. In the vectormethod, a target gene to be analyzed is incorporated into a vector, andthe full-length sequence of the gene obtained after the proliferation isthen determined using a sequencer. However, the vector method isproblematic in that it needs a culture operation, and also in that asequencer should be used to analyze the full length of a gene.

In recent years, a high speed sequencer used in gene analysis has beendeveloped, and with the development of this sequencer, mate-pair methodas a gene analysis means has attracted attention.

FIG. 1 schematically shows the outline of gene analysis using themate-pair method. In the mate-pair method, a nucleotide sequence forligation (a restriction enzyme recognition site) is attached to the bothends of the target gene to be analyzed, and the target gene is thencircularized. Then, a part including 15 nucleotides or more, andpreferably 25 nucleotides or more to several tens of nucleotides orless, of both sides flanking to the restriction enzyme recognition siteis cleaved from the circularized gene, generally using a type IIrestriction enzyme. The part is amplified by PCR, and the nucleotidesequence of the cleaved partial gene is then determined. Thereby, thesequences of both ends of the target gene are determined, and the targetgene can be then identified using known sequence data. Mate pair meansthe sequence data of a pair of nucleotide sequences obtained by readingboth ends of a single DNA fragment.

Practically used methods of cleaving a given number of nucleotides froma gene include: a method of cutting sites apart from the recognitionsite using a type II restriction enzyme to cleave a given number ofnucleotides out; and a method comprising physically cutting circular DNAusing Sonication or the like, recovering a cleaved fragment with biotinattached to the linker, then amplifying the fragment by PCR, and thendetermining the sequence of the amplified PCR product.

The mate-pair method can, therefore, identify a known gene by reading acertain length of nucleotide sequence including the both sides flankingto the ligation site in a gene circularized by ligating the both ends ofthe DNA. Basically, if partial nucleotide sequences of the head and tailportions of a gene were read, these sequences would allow a reliablediscrimination among individual genes. Accordingly, the mate-pair methodhas been adopted as a reliable and simple gene analysis method (NonPatent Literatures 1 and 2). Moreover, the mate-pair method has beenapplied to the next generation sequence analysis, and thus it has becomeincreasingly important together with the emergence of a high speedsequencer.

However, when a DNA is circularized for gene analysis according to themate-pair method, besides the self-circularization of a single gene or asingle DNA (a single molecule), the circularization of a plurality ofDNAs (a plurality of molecules) and the linear binding of a plurality ofmolecules (two or more molecules) also take place. A linear moleculeconsisting of a plurality of molecules can be separated and eliminatedfrom a circular molecule by the subsequent operations. On the otherhand, a circular molecule consisting of a plurality of molecules cannotbe separated from a circular molecule consisting of a single molecule,and it becomes a contaminant. A circular product consisting of aplurality of molecules inhibits individual gene analyses andsignificantly decreases analytical specificity for the followingreasons. Specifically, as shown in FIG. 2, when three types of cDNAs areto be self-circularized, if only single-molecule DNA is circularized asshown in (B), a gene can be specified using precise sequences accordingto the mate-pair method. However, other than the circularization or asingle molecule as shown in (B), an uncircularized linear product may begenerated as shown in (C), or two or more cDNAs may be circularized asshown in (D). In the case of (C), the linear product can be eliminatedwith DNA exonuclease. However, in the case of (D), a circularizedproduct consisting of a plurality of cDNAs is recognized as acircularized molecule, and thus, it cannot be eliminated and becomes acontaminant in the gene analysis according to the mate-pair method.

The gene analysis according to the mate-pair method intends to identifya target gene based on the nucleotide sequences of both ends of thetarget gene. Specifically, a ligation adapter for circularization isattached to both ends of individual genes, and the two adapter sites arethen ligated to each other to circularize the gene. Then, a partincluding a certain number of nucleotides at both sides with the adaptersite being the center is cleaved from the gene. Consequently, the genecan be identified by analyzing the nucleotide sequence of a portion fromeach end of the original gene. Hence, a circularized product of aplurality of molecules has a plurality of adapter sites, and the twoends attached to an adapter are ends of different genes. Accordingly, asdescribed above, since gene analysis is carried out by cleaving a partincluding nucleotide sequences of a given number of nucleotides of theboth ends attached to the adapter, with the adapter being the center,according to either one of the above described two methods, the genefragment for the analysis obtained from circularized products of aplurality of molecules comprises ends of different genes. Thus, theanalysis of a single gone cannot be carried out.

As such, in the gene analysis according to the mate-pair method, thepresence of a circularized product of a plurality of molecules inhibitseach gene analysis.

The probability of circularization of a plurality of DNA moleculesgenerally ranges from few to dozen percent, depending on the methodapplied. In the analysis of a known gene, they are recognized asabnormal nucleotide sequences and thus, elimination of them from thesequence to be analyzed is usually possible. Hence, it causes only aslight decrease in accuracy, although the operation becomes complicated.However, in a case in which the mate-pair method is used to detect thepresence of an abnormal gene, such as a fusion gene, in a group ofnormal genes, if a plurality of normal genes are circularized, it isdetermined that abnormal genes are present. As a result, it becomesimpossible to accurately confirm the presence of an abnormal gene suchas a fusion gene.

The fusion gene is a gene with a novel function that is constructed bybinding a plurality of (two) genes to each other. For example,abnormalities in chromosome structure, such as deletion, overlapping,recombination and translocation, are found in a cancer cell. When thecleavage and ligation of a gene occur at a DNA level and a structuralgene is present at each cleavage point, a fusion gene is formed.

In general, a fusion gene is lethal or senseless to cells, and it doesnot cause clinical problems in many cases. However, when cell growth isabnormally promoted as a result that a fusion protein generated fromsuch a fusion gone inhibits the control of the cell growth, it causesclinical problems such as tumor formation.

It had been considered that the fusion gene is mainly expressed inhematopoietic tumors. In recent years, however, it has been expectedthat the fusion gene would be also associated with epithelial solidtumors (Non Patent Literature 3). Among such solid cancers, responsiblefusion genes have been discovered from prostatic cancer and lung cancer(Non Patent Literatures 4 and 5).

From these findings, the analysis of a fusion gene, namely, confirmationof the presence of a fusion gene, has attracted attention as a novelmethod for diagnosing tumors (cancers) and the like. Specifically, bydetecting a known fusion gene that has been known as corresponding topathologic conditions, it becomes possible to make a rapid diagnosis ofthe pathologic conditions. Furthermore, the discovery of a novel fusiongene leads co the discovery of drug discovery targets.

On the other hand, conventional chromosome analyses performed on solidtumors had had a certain limit, and it had been extremely difficult toanalyze and/or confirm a fusion gene. Recently, novel methods, such asthe cDNA functional expression analysis method according to Mano et al.,have been developed. However, these techniques have been stillinsufficient in terms of complicated operations, problems regardingaccuracy, etc. (Patent Literature 1). In addition, various types ofnext-generation high speed gene sequencers have been recently developed.Thus, high speed sequence analysis of genes has significantlyprogressed, and gene analysis in a short time has been realizing. Hence,searching for fusion genes have been started according to high-speedand/or high-scale nucleotide sequence analysis of tumor genomes and/orgenes (Non Patent Literature 6).

In order to identify a fusion gene by sequence analysis using themate-pair method, it is essential to reliably produce single circularDNA from a single cDNA molecule. A schematic view of the analysis of afusion gene according to the mate-pair method is shown in FIG. 3. Thereis a case, however, in which single circular DNA may be formed from aplurality of cDNA molecules, as shown in FIG. 4. Thus, when sequenceanalysis is carried out according to the mate-pair method, the resultthat a normal gene appears as a fusion gene may be obtained. If thisgene is eliminated for the reason that it is not present in aconventional gene sequence, a fusion gene is also eliminated. As aresult, it becomes substantially impossible to confirm the presence of afusion gene.

When a fusion gene is to be detected by the sequence analysis accordingto the mate-pair method, it is essential to eliminate circularized cDNAof a plurality of genes.

LIST OF CITATION Patent Literature

-   Patent Literature 1: Japanese Patent No. 4303303 Non Patent    Literature-   Non Patent Literature 1: Tanpakushitsu Kakusan Koso, August 2009,    (1233-1247, 1271-1275)-   Non Patent Literature 2: “Shikkan Idenshi no Tansaku to Chokosoku    Sequence,” Jikken Igaku extra edition, Vol. 27, No. 12 (2009,    113(1929)-143(1959))-   Non Patent Literature 3: Mitelman et al., 2004, Nature Genetics,    Vol. 36, No. 4, pp. 331-334-   Non Patent Literature 4: Chinnaiyan et al., 2005, Science, Vol. 310,    pp. 644-648-   Non Patent Literature 5: Soda et al., 2007, Nature, Vol. 448, pp.    561-566-   Non Patent Literature 6: Bashir et al., April 2008, PLoS    Computational Biology, Vol. 4, Issue 4, e1000051

SUMMARY OF INVENTION Technical Problem

It is desired to discover a method for reliably producing a circular DNAmolecule formed only from a single-molecule DNA. That is to say, it isan object of the present invention to provide a method capable ofproducing a single circularized DNA from a single DNA.

Solution to Problem

The present inventors have found that a circularized DNA consisting onlyof a single molecule can be formed by introducing an adapter having aspecific structure comprising a specific sequence recognition site intoa single DNA molecule and then performing two-step ligation. Theinventors have found a method for reliably circularizing onlysingle-molecule DNA, wherein circularization of multiple moleculesconsisting of a plurality of DNAs does not take place.

In a first embodiment, the present invention provides a method forproducing a circular DNA which consists of a circular DNA formed fromsingle-molecule DNA and which does not comprise circular DNA formed froma multiple-molecule DNA, the method comprising the following steps:

1) a step of binding an adapter (A) for a first circularization to oneend of each DNA molecule of interest, and binding to the other endthereof an adapter (B) for a second circularization comprising anadapter (b) and the adapter (A), wherein the adapter (B) binds to theDNA molecule via the adapter (b) side, such that the adapter (A) in theadapter (B) is located outside the bond of the DNA molecule to theadapter (b), wherein

the adapter (A) comprises a cleavage site generating a cleavage endnon-specifically binding to all of the cleavage ends of the adapter (A),and

the adapter (b) comprises a cleavage site generating a cleavage endspecifically binding only to a cleavage end from the same adapter (b);

2) a first cleavage step of cleaving the DNA molecule obtained instep 1) at the cleavage site of the adapter (A);3) a first circularization step of binding both ends of the DNA moleculeobtained in step 2) to circularize the DNA molecule;4) a step of eliminating an uncircularized linear single-molecule and amultiple-molecule-bound DNA in step 3);5) a second cleavage step of cleaving the circular DNA molecule obtainedin step 3) and step 4) at the cleavage site of the adapter (b); and6) a second circularization step of binding both ends of the DNAmolecule obtained in step 5) to circularize the DNA molecule.

In the second circularization step, since the adapter (b) specificallybinds to a cleavage end from the same adapter (b), linearization causedby circularization of a plurality of DNA molecules substantially doesnot take place, and the DNA to be circularized is a single-molecule DNA.At this time, the uncircularized linear DNA molecules need to beeliminated, as described above.

The linear DNAs that are not circularized in step 6) consist of: a traceamount of single-molecule DNA, which have failed to rebind the portion(b) cleaved in step 5); and a major amount of single-molecule ormultiple-molecule DNAs, which have be circularized with a plurality ofDNA molecules in step 3) and have failed to rebind each adapter (b)after the cleavage of the adapter in step 5).

In the first embodiment of the present invention, the adapter (A) ispreferably double-stranded DNA comprising a restriction enzyme siterecognizing a sequence whose cleavage ends are complementary to eachother, and it is, for example, double-stranded DNA comprising arestriction enzyme site recognizing a palindromic sequence.

In addition, in a second embodiment, the present invention provides amethod for producing a single circular DNA which consists of a circularDNA formed from a single-molecule DNA and which does not comprise acircular DNA formed from a multiple-molecule DNA, the method comprisingthe following steps:

1) a step of binding an adapter (A) for a first circularization to oneend of each DNA molecule of interest, and binding to the other endthereof an adapter (B) for a second circularization comprising anadapter (b) and the adapter (A), wherein the adapter (B) binds to theDNA molecule via the adapter (b) side, such that the adapter (A) in theadapter (B) is located outside the bond of the DNA molecule to theadapter (b), wherein

the adapter (A) is a double-stranded DNA comprising a restriction enzymesite recognizing a palindromic sequence, and

the adapter (B) comprises the adapter (b) and the adapter (A), whereinthe adapter (b) is a double-stranded DNA comprising identicalnick-generating enzyme recognition sites or restriction enzyme sitesbeing reversely oriented to each other, wherein the adapter (b) is adouble-stranded DNA comprising a double-stranded DNA sequence having aunique sequence for each adapter (b) between the identicalnick-generating enzyme recognition sites or restriction enzyme sitesbeing reversely oriented to each other, and if the adapter (b) iscleaved at the identical nick-generating enzyme recognition sites orrestriction enzyme sites being reversely oriented to each other, thecleavage site rebinds only to the cleavage site from the same adapter(b);

2) a first cleavage step of cleaving the DNA molecule obtained instep 1) with a restriction enzyme recognizing the restriction enzymesite comprised in the adapter (A);3) a first circularization step of ligating both ends of the DNAmolecule obtained in step 2) to circularize the DNA molecule;4) a step of eliminating an uncircularized linear single-molecule and amultiple-molecule-bound DNA in step 3);5) a second cleavage step of cleaving the circular DNA molecule obtainedin step 3) and step 4) with a nick-generating enzyme or a restrictionenzyme recognizing the nick-generating enzyme recognition sites orrestriction enzyme sites of the adapter (b); and6) a second circularization step of ligating both ends of the DNAmolecule obtained in step 5) to circularize the DNA molecule.

Preferably, the above described second embodiment of the presentinvention further comprises a step of digesting with endonuclease acircularized DNA formed by binding the adapters (b) via cleavagesequence sites being partially different from each other (missannealing) and circularization in the above described step 6). Thereby,circular DNA consisting of a plurality of molecules can be more reliablyeliminated.

In the second embodiment of the present invention, preferably,

the adapter (A) is a double-stranded DNA comprising a restriction enzymesite X recognizing a palindromic sequence, and

the adapter (B) is a double-stranded DNA consisting of two DNA strandscomplementary to each other having the following structure y₁-Y-y₂-X:

whereinX represents a double-stranded DNA being a restriction enzyme siterecognizing a palindromic sequence;y₁ and y₂ represent double-stranded DNAs respectively comprisingidentical nick-generating enzyme recognition sites or restriction enzymesites being reversely oriented to each other;Y represents a double-stranded DNA sequence having a unique sequence foreach DNA molecule to be circularized, n represents an integer of 1 to40, N₁ to N_(n), which may be the same or different, each represent adeoxyribonucleotide selected from the group consisting of dAMP, dCMP,dGMP and dTMP, and N′₁ to N′_(n), represent the followingdeoxyribonucleotides respectively corresponding to the N₁ to N_(n),

TABLE 1 N_(k) N′_(k) dAMP dTMP dCMP dGMP dGMP dCMP dTMP dAMPwherein k represents an integer from 1 to n.

In the above structure, n is 1 to 40, preferably 4 to 15, and morepreferably 5 to 10. However, even if n is more than 40, the method ofthe present invention can be carried out.

In this embodiment, preferably, y₁ and y₂ respectively comprise thenick-generating enzyme recognition sites, and more preferably, thenick-generating enzyme is Nb.BtsI that recognizes 6 nucleotides, orNt.BsqQI that recognizes 7 nucleotides. Alternatively, y₁ and y₂ mayrespectively comprise the restriction enzyme sites.

In a third embodiment, the present invention provides an adapter forproducing a circular DNA, consisting of two DNA strands complementary toeach other having the following structure y₁-Y-y₂-X:

whereinX represents a double-stranded DNA that is a restriction enzyme siterecognizing a palindromic sequence;y₁ and y₂ represent double-stranded DNAs respectively comprisingidentical nick-generating enzyme recognition sites or restriction enzymesites being reversely oriented to each other;Y represents a double-stranded DNA sequence having a unique sequence foreach DNA molecule to be circularized,n represents an integer of 1 to 40, N₁ to N_(n), which may be the sameor different, each represent a deoxyribonucleotide selected from thegroup consisting of dAMP, dCMP, dGMP and dTMP, and N′₁ to N′_(n)represent the following deoxyribonucleotides respectively correspondingto the N₁ to N_(n),

TABLE 2 N_(k) N′_(k) dAMP dTMP dCMP dGMP dGMP dCMP dTMP dAMPwherein k represents an integer from 1 to n.

In the above structure, n is 1 to 40, preferably 4 to 15, and morepreferably 5 to 10. However, even if n is more than 40, the method ofthe present invention can be carried out.

The above described adapter in the third embodiment of the presentinvention corresponds to the adapter (B) in the second embodiment of thepresent invention, and it is preferably used in the method of thepresent invention for producing a circular DNA which consists of acircular DNA formed from a single-molecule DNA and which dies notcomprise a circular DNA formed from a multiple-molecule DNA.

In the third embodiment of the present invention, preferably, y₁ and y₂respectively comprise nick-generating enzyme recognition sites, and morepreferably, the nick-generating enzyme is Nb.BtsI that recognize 6nucleotides, or Nt.BsqQI that recognizes 7 nucleotides. Alternatively,y₁ and y₂ may respectively comprise restriction enzyme sites.

In a fourth embodiment, the present invention further provides a kit forproducing a circular DNA comprising the adapter according to the abovedescribed third embodiment (which is also simply referred to as adapter(B)) and an adapter consisting of a double-stranded DNA comprising arestriction enzyme site identical to the restriction enzyme siterepresented by X comprised in the adapter (B) (which is also simplyreferred to as adapter (A)).

In a fifth embodiment, the present invention also provides a method forproducing a cDNA library, comprising using the method according to theabove described first or second embodiment.

In a sixth embodiment, the present invention further provides a methodfor identifying a gene by subjecting a circular DNA molecule obtained byusing the method according to the above described first or secondembodiment to a mate-pair method, the method comprising the followingsteps:

1) a step of reading nucleotide sequences, each consisting of 15 to 600nucleotides, flanking to both sides of an adapter (B) in a circular DNAmolecule obtained by using the method according to the above describedfirst or second embodiment; and2) a step of identifying a gene contained in the circular DNA moleculeby comparing the read nucleotide sequences with the sequences of bothends of a known gene.

The nucleotide sequence to be read consists of preferably 15 to 100nucleotides, and more preferably 25 to 35 nucleotides. However, themethod of the present invention can also be carried out by reading anucleotide sequence consisting of 600 or more nucleotides.

In a seventh embodiment, the present invention further provides a methodfor detecting a fusion gene by subjecting a circular DNA moleculeobtained by using the method according to the above described first orsecond embodiment to a mate-pair method, the method comprising thefollowing steps:

1) a step of reading nucleotide sequences, each consisting of 15 to 600nucleotides, flanking to both sides of an adapter (B) in a circular DNAmolecule obtained by using the method according to the above describedfirst or second embodiment; and2) a step of comparing the read nucleotide sequences with the sequencesof both ends of a known gene, wherein if the genes of both endscorrespond to known different genes, the gene comprised in the circularDNA molecule is identified to be a fusion gene.

The nucleotide sequence to be read consists of preferably 15 to 100nucleotides, and more preferably 25 to 35 nucleotides. However, themethod of the present invention can also be carried out by reading anucleotide sequence consisting of 600 or more nucleotides.

In the seventh embodiment of the present invention, when the sequencesof both sides flanking to both sides of the adapter (B) correspond tothe ends of both sides of a known fusion gene, the known fusion gene isdetected. When the relationship between the known fusion gene and adisease has been known, there is provided a method for detecting adisease characterized by the expression of the fusion gene detected bythe method of the seventh embodiment, wherein the fusion gene is used asa marker.

On the other hand, when the sequences flanking to both sides of theadapter (B) correspond to the terminal sequences of different genes anddo not correspond to the ends of both sides of a known fusion gene, thegene contained in the circular DNA molecule is identified to be a novelfusion gene. In this case, the detected novel fusion gene is preferablyused in drug discovery screening.

Advantageous Effects of Invention

Using the adapter and method of the present invention, the accuracy ofgene analysis is significantly improved. In addition, unconventionalhigh-accuracy mate-pair analysis becomes possible, and thus, the adapterand method of the present invention are provided as tools extremelyuseful for genomic analysis. In particular, by applying the method ofthe present invention to the production of a cDNA library, it is highlylikely to discover a novel fusion gene. That is to say, by reliablyachieving circularization of a single DNA molecule, it becomes possibleto develop novel diagnostic tool and/or method.

According to the present invention, a method for circularizing only asingle-molecule DNA, wherein circularization of a plurality of moleculesis substantially prevented with regard to circularization of DNA, isprovided, sc that problems regarding contamination in gene analysis suchas mate-pair analysis can be solved, and so that high-accuracy analysisbecomes possible. Moreover, by applying the method of the presentinvention to detection and/or analysis of a fusion gene, high-accuracyanalysis of a fusion gene becomes possible, and a useful diagnostic toolcan be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically shows an outline of the gene analysis according tothe mate-pair method.

FIG. 2 schematically shows problems with the gene analysis according tothe mate-pair method.

FIG. 3 is a schematic view showing the analysis of a fusion geneaccording to the mate-pair method.

FIG. 4 schematically shows problems with the analysis of a fusion geneaccording to the mate-pair method.

FIG. 5 schematically shows a first cleavage step in the two-stepcircularization method according to the present invention.

FIG. 6 schematically shows first circularization, second cleavage, andsecond circularization steps in the two-step circularization methodaccording to the present invention.

FIG. 7 is a schematic view showing application of the method of thepresent invention to cDNA synthesized from mRNA.

FIG. 8 is a schematic view showing application of the method of thepresent invention to a genomic library.

FIG. 9 shows the experimental results of Example 4, which demonstratethat the production amount of a circularized DNA consisting of aplurality of molecules is decreased by application of the method of thepresent invention.

DESCRIPTION OF EMBODIMENTS Regarding a First Embodiment of the PresentInvention

In a first embodiment, the present invention provides a method forproducing a circular DNA which consists of a circular DNA formed from asingle-molecule DNA and which does not comprise a circular DNA formedfrom a multiple-molecule DNA, the method comprising the following steps:

1) a step of binding an adapter (A) for a first circularization to oneend of each DNA molecule of interest, and binding to the other endthereof an adapter (B) used for a second circularization comprising anadapter (b and the adapter (A), wherein the adapter (B) binds to the DNAmolecule via the adapter (b) side, such that the adapter (A) in theadapter (B) is located outside the bond of the DNA molecule to theadapter (b), wherein

the adapter (A) comprises a cleavage site generating a cleavage endnon-specifically binding to all of the cleavage ends of the adapter (A),and

the adapter (D) comprises a cleavage site generating a cleavage endspecifically binding only to a cleavage end from the same adapter (b);

2) a first cleavage step of cleaving the DNA molecule obtained instep 1) at the cleavage site of the adapter (A);3) a first circularization step of binding both ends of the DNA moleculeobtained in step 2) to circularization the DNA molecule;4) a step of eliminating an uncircularized linear single-molecule and amultiple-molecule-bound DNA in step 3);5) a second cleavage step of cleaving the circular DNA molecule obtainedin step 3) and step 4) at the cleavage site of the adapter (b); and6) a second circularization step of binding both ends of the DNAmolecule obtained in step 5) to circularization the DNA molecule.

The above described first embodiment of the present invention relates toa technique of circularizing substantially only a single gene (DNA),such that circularization of a plurality of genes (DNAs) can beprevented.

Specifically, the method of the first embodiment of the presentinvention is characterized in that it comprises two-stepcircularization.

Hereinafter, the method of the first embodiment will be described in theorder of steps.

Step 1) is a step of binding an adapter (A) for a first circularizationto one end of each DNA molecule of interest, and binding to the otherend thereof an adapter (B) used for a second circularization comprisingan adapter (b) and the adapter (A). Herein, the adapter (B) binds to theDNA molecule via the adapter (b) side, such that the adapter (A) in theadapter (B) is located outside the bond of the DNA molecule to theadapter (b). Specifically, the adapter (A) is allowed to bind to one endof each DNA molecule of interest, and the adapter (B) is allowed to bindto the other end, so that the adapter (A) in the adapter (B) is locatedon the side closer to the end than the adapter (b) is. As a result, theadapters (A) are located on both ends of each DNA molecule of interest.

It is to be noted that the adapter (A) comprises a cleavage sitegenerating a cleavage end non-specifically binding to all of thecleavage ends of the adapter (A), and that the adapter (b) comprises acleavage site generating a cleavage end specifically binding only to acleavage end from the same adapter (b). That is to say, the adapter (A)does not exhibit binding specificity and enables non-specific bindingbetween all cleavage ends from the adapter (A). In contrast, the adapter(b) has a constitution that is different depending on each DNA moleculeof interest, and it is able to rebind to only cleavage ends from thesame adapter (b) and is not able to rebind to cleavage ends from adifferent adapter (b).

Step 2) is a first cleavage step of cleaving the DNA molecule obtainedin step 1) at the cleavage site of the adapter (A).

Since the adapters (A) bind to both ends of the DNA molecule obtained instep 1), cleavage ends from the adapters (A) are generated on both endsof each DNA molecule as a result of the first cleavage step.

Step 3) is a first circularization step of binding both ends of the DNAmolecule obtained in step 2) to circularize the DNA molecule. The firstcircularization is circularization with the adapters (A), and suchcircularization occurs as a result that cleavage ends from the adapters(A) generated on both ends of each DNA molecule bind to each other.Herein, the adapter (B), as well as the adapters (A), is incorporatedinto the circular molecule during the first circularization. Asdescribed above, the function of the adapter (A) does not havespecificity, and does not have selectivity by the rebinding of both endsof the DNA molecule of interest. Thus, not only the binding of both endsof a single DNA molecule at the adapter (A) portions, butcircularization by the binding of both ends of each of a plurality ofDNA molecules, also takes place. That is, because of the function of theadapter (A), circularization of a plurality of molecules, as well assingle-molecule circularization, takes place. A circularized productfrom such a plurality of molecules naturally comprises a plurality ofthe adapters (B).

Step 4) is a step of eliminating an uncircularized linearsingle-molecule and a multiple-molecule-bound DNA in step 3).

Step 5) is a second cleavage step of cleaving the circular DNA moleculeobtained in step 3) and step 4) at the cleavage site of the adapter (b).That is to say, cleavage is carried out at the adapter (b) incorporatedinto each DNA molecule that has been circularized in step 3), such thata linear molecule is generated. By the second cleavage step, cleavageends from the adapters (b) are generated on both ends of each DNAmolecule. In this step, the cleavage end generated as a result of thecleavage substantially cannot bind to a cleavage end from anothercircular molecule.

Step 6) is a second circularization step of binding both ends of the DNAmolecule obtained in step 5) to circularize the DNA molecule. That is tosay, the cleavage ends from the adapters (b) are present on both ends ofeach DNA molecule obtained in step 5). As described above, the adapter(b) has a constitution that is different depending on each DNA moleculeof interest, and it is able to rebind to only cleavage ends from thesame adapter (b) and is not able to rebind to cleavage ends from adifferent adapter (b). Thus, as a result of the binding of cleavage endsfrom the adapters (b) on both ends of the same DNA molecule, secondcircularization takes place. Because of the specificity of the adapter(b), only single-molecule DNA of interest is circularized. That is, theadapter (b) has binding specificity, and when each adapter (b) portionis cleaved, it rebinds only to a portion, to which it has bound in theoriginal circularized product, but it cannot rebind to other portions,upon rebinding circularization. Accordingly, when rebinding andcircularization are carried out in the case of circularization of aplurality of molecules, only the rebinding and circularization in aplurality of the original molecules are possible. As described later,this probability is extremely low, and it is substantially 0 (zero). Theprobability of circularization of a single molecule is generally highbecause of its concentration, and as a result, only circularization of asingle molecule takes place substantially. Uncircularized linearmolecules can be separated and eliminated by allowing exonuclease to acton them.

As far as the adapters (A) and (b) perform the above-mentionedfunctions, the structures thereof are not particularly limited. Whenthese adapters are double-stranded DNAs, an example of the adapter (A)is an adapter comprising a restriction enzyme site recognizing apalindromic structure, whereas an example of the adapter (b) isdouble-stranded DNA consisting of nucleic acids of at least 4nucleotides, and preferably 5 or more nucleotides, which arecomplementary to each other, wherein the double-stranded DNA generatessingle-stranded ends complementary to each other, as a result ofcleavage. However, the adapters (A) and (b) are not limited thereto. Forinstance, another example of the adapter (b) may be a substance thatbecomes a key and a keyhole as a result of cleavage. In a word, when theadapter (b) is cleaved and is then rebound, it may be an adapter thatcan rebind substantially only to the adapter before the cleavage.However, even in the case of a cleavage site having a non-palindromicsequence, the method of the present invention can be applied. In such acase, a configuration, in which an upstream adapter (A1) isdistinguished from a downstream adapter (A2) and in which (A1) binds to(A2), is adopted. In this case, it is anticipated that the efficiency ofsingle-molecule circular DNA at the first step will be slightlyincreased, in comparison with the use of an ordinary adapter (A).

According to the first embodiment of the present invention, circularizedproducts from a plurality of genes (a plurality of molecules), whichbecome contaminants, can be eliminated from, for example, gene analysisand gene identification according to the mate-pair method, and itbecomes possible to produce circularized products only from a singlemolecule (a single gene).

Regarding a Second Embodiment of the Present Invention

The a second embodiment of the present invention provides a method forproducing a circular DNA which consists of a circular DNA formed from asingle-molecule DNA and which does not comprise a circular DNA formedfrom a multiple-molecule DNA, the method comprising the following steps:

1) a step of binding an adapter (A) for a first circularization to oneend of each DNA molecule of interest, and binding to the other endthereof an adapter (B) for a second circularization comprising anadapter (b) and the adapter (A), wherein the adapter (B) binds to theDNA molecule via the adapter (b) side, such that the adapter (A) in theadapter (B) is located outside the bond of the DNA molecule to theadapter (b), wherein

the adapter (A) is a double-stranded DNA comprising a restriction enzymesite recognizing a palindromic sequence, and

the adapter (B) comprises the adapter (b) and the adapter (A), whereinthe adapter (b) is a double-stranded DNA comprising identicalnick-generating enzyme recognition sites or restriction enzyme sitesbeing reversely oriented to each other, wherein the adapter (b) is adouble-stranded DNA comprising a double-stranded DNA sequence having aunique sequence for each adapter (b) between the identicalnick-generating enzyme recognition sites or restriction enzyme sitesbeing reversely oriented to each other, and if the adapter (b) iscleaved at the identical nick-generating enzyme recognition sites orrestriction enzyme sites being reversely oriented to each other, thecleavage site rebinds only to the cleavage site from the same adapter(b);

2) a first cleavage step of cleaving the DNA molecule obtained instep 1) with a restriction enzyme recognizing the restriction enzymesite comprised in the adapter (A);3) a first circularization step of ligating both ends of the DNAmolecule obtained in step 2) to circularize the DNA molecule;4) a step of eliminating an uncircularized linear single-molecule and amultiple-molecule-bound DNA in step 3);5) a second cleavage step of cleaving the circular DNA molecule obtainedin step 3) and step 4) with a nick-generating enzyme or a restrictionenzyme recognizing the nick-generating enzyme recognition sites orrestriction enzyme sites of the adapter (b); and6) a second circularization step of ligating both ends of the DNAmolecule obtained in step 5) to circularize the DNA molecule.

In the second embodiment of the present invention, it is preferable thatthe production method further comprise a step of digesting withendonuclease a circularized DNA formed by binding the adapters (b) viacleavage sequence sites being partially different from each other andcircularization in the above described step 6).

Hereafter, the second embodiment of the present invention will bedescribed along the steps thereof, while referring to FIGS. 5 and 6.

As schematically shown in FIG. 5, step 1) is a step of binding anadapter (A) for a first circularization to one end of each DNA moleculeof interest, and binding to the other end thereof an adapter (B) usedfor a second circularization comprising an adapter (b) and the adapter(A). Herein, the adapter (B) binds to the DNA molecule via the adapter(b) side, such that the adapter (A) in the adapter (B) is locatedoutside the bond of the DNA molecule to the adapter (b).

Herein, the adapter (A) is a double-stranded DNA comprising arestriction enzyme site recognizing a palindromic sequence; and theadapter (B) comprises the adapter (b) and the adapter (A), wherein theadapter (b) is a double-stranded DNA comprising identicalnick-generating enzyme recognition sites or restriction enzyme sitesbeing reversely oriented to each other, wherein the adapter (b) is adouble-stranded DNA comprising a double-stranded DNA sequence having aunique sequence for each adapter (b) between the identicalnick-generating enzyme recognition sites or restriction enzyme sitesbeing reversely oriented to each other, and if the adapter (b) iscleaved at the identical nick-generating enzyme recognition sites orrestriction enzyme sites being reversely oriented to each other, thecleavage site rebinds only to the cleavage site from the same adapter(b).

The adapters (A) and (b) will be described in detail below.

Adapter (A): This adapter is a sequence for cleaving the DNA moleculewith restriction enzymes in the first cleavage and then performing geneligation to produce circular DNA. Accordingly, the adapter (A) maycomprise any type of restriction enzyme site, as long as the restrictionenzyme site recognizes a palindromic sequence. Preferably, the adapter(A) comprises a restriction enzyme site recognizing a rare gene sequencethat hardly cleaves the DNA of interest. Examples of the restrictionenzyme site comprised in the adapter (A) include NotI and EcoRI.

Adapter (b): This adapter is a double-stranded DNA comprising identicalnick-generating enzyme recognition sites or restriction enzyme sitesbeing reversely oriented to each other, wherein it comprises adouble-stranded DNA sequence having a unique sequence for each adapter(b) between the identical nick-generating enzyme recognition sites orrestriction enzyme sites being reversely oriented to each other. Thisnick-generating enzyme recognition site or restriction enzyme site isused to cleave the DNA molecule in the second cleavage so as to form agene overhang specific to each molecule. The nick-generating enzymecleaves only one DNA strand by nick cleavage, whereas the restrictionenzyme cleaves two DNA strands simultaneously. Both of these recognitionsites can be introduced. When the adapter (b) is cleaved at theidentical nick-generating enzyme recognition sites or restriction enzymesites being reversely oriented to each other, cleavage sites aregenerated. The cleavage sites rebind only to cleavage sites from thesame adapter (b). In the case of using the nick-generating enzyme, thelength of the cleavage site is not limited. In the case of using therestriction enzyme, however, the length of the cleavage site is limited,as shown in the following examples. Thus, in order to arbitrarily adjustthe length of the cleavage site, the nick-generating enzyme ispreferable.

Specific examples of each of the nick-generating enzyme recognition siteand the restriction enzyme site will be given below.

In Case of Nick-Generating Enzyme Recognition Site

Restriction Enzyme Case 1 [Formula 5]

Restriction Enzyme Case 2

Restriction Enzyme Case 3

In the case of this form, it is necessary to prepare two sites of Nsequences that are equivalent to each other, by applying a method ofextending primers from a loop structure, etc.

Subsequently, a cleavage site, which is generated when the adapter (b)is cleaved at the identical nick-generating enzyme recognition sites orrestriction enzyme sites being reversely oriented to each other, will bedescribed.

The cleavage site is a portion corresponding to a gene overhang specificto each molecule. As described above, when the cleavage site is producedwith the restriction enzyme, the number of nucleotides is limited. Onthe other hand, when the cleavage site is produced with thenick-generating enzyme, the length of the cleavage site can bearbitrarily set. In order to achieve high specificity, a greater numberof nucleotides constituting such a cleavage site is appropriate. Incontrast, it the cleavage site is too long, it is highly likely thatgene ligation will occur due to mismatch. When complexity is increased,such as the case of genome, it is desirable that the number ofnucleotides constituting the cleavage site should be set to be greater,and at the same time, a step of digestion with endonuclease should beadded to prevent the possibility of mismatch ligation (miss annealing).It is to be noted that a random combination is applied to the sequenceof a cleavage site, and each different cleavage site is designed foreach DNA molecule.

A specific example will be given below, while the number of nucleotidesis set at “5”. As an example, an adapter (B) having the followingsequence is produced by nucleic acid synthesis, and then, it can be usedin the method of the present invention.

Step 2) is a first cleavage step of cleaving the DNA molecule obtainedin step 1) with a restriction enzyme recognizing the restriction enzymesite comprised in the adapter (A) (FIG. 5).

Step 3) is a first circularization step of ligating both ends of the DNAmolecule obtained in step 2) to circularize the DNA molecule (FIG. 6;first circularization).

Step 4) is a step of eliminating an uncircularized linearsingle-molecule and a multiple-molecule-bound DNA in step 3).

Step 5) is a second cleavage step of cleaving the circular DNA moleculeobtained in step 3) and step 4) with a nick-generating enzyme or arestriction enzyme recognizing the nick-generating enzyme recognitionsites or restriction enzyme sites of the adapter. (b)

Step 6) is a second circularization step of ligating both ends of theDNA molecule obtained in step 5) to circularize the DNA molecule (FIG.6; cleavage and recircularization). In this step, because of thespecificity of the sequence of the cleavage site, only the cleavagesites from the adapter (b) bound to the same DNA can bind to each other.

In the second embodiment of the present invention, preferably, theadapter (A) is a double-stranded DNA comprising a restriction enzymesite X recognizing a palindromic sequence, and the adapter (B) is adouble-stranded DNA consisting of two DNA strands complementary to eachother having the following structure y₁-Y-y₂-X:

whereinX represents a double-stranded DNA being a restriction enzyme siterecognizing a palindromic sequence;y₁ and y₂ represents double-stranded DNAs respectively comprisingidentical nick-generating enzyme recognition sites or restriction enzymesites being reversely oriented to each other;Y represents a double-stranded DNA sequence having a unique sequence foreach DNA molecule to be circularized,n represents an integer of 1 to 40, N₁ to N_(n), which may be the sameor different, each represent a deoxyribonucleotide selected from thegroup consisting of dAMP, dCMP, dGMP and dTMP, and N′₁ to N′_(n),represent the following deoxyribonucleotides respectively correspondingto the N₁ to N_(n),

TABLE 3 N_(k) N′_(k) dAMP dTMP dCMP dGMP dGMP dCMP dTMP dAMPwherein k represents an integer from 1 to n.

In the above structure, n is 1 to 40, preferably 4 to 15, and morepreferably 5 to 10. However, even if n is more than 40, the method ofthe present invention can be carried out.

The Y site consists of a pair of nucleotide sequences, which arerandomly combined. For example, if Y is constituted with 8 nucleotidesequences, 4⁸ combinations are obtained. Thereby, pair formation incircularization can be specified (selected), and rebinding only to theoriginal pair becomes possible. As a result, only a single DNA can berecircularized, and since the Y portion of another molecule his adifferent structure, it cannot rebind thereto.

Since 4 types of nucleic acids are combined in a repeated permutationmanner, if an N number of nucleic acids are present in the Y portion,the number of the sequences can be 4^(N). Accordingly, if N is 5,4⁵=1,016 types of nucleic acids are present in the Y portion. If N is 7,16,000 or more nucleic acids are present in the Y portion. Thus, N=7 issubstantially sufficient. However, there is no problem, even if thenumber of nucleic acids is 8 or greater. When N is a great number, evenif the nucleic acids are partially different, hybridization and ligationcan be carried out. When the nucleic acids contain such mismatch, theycan be digested using endonuclease. As a method for producing the Yportion of the adapter (B), the Y portion can be basically produced bydetermining N (the number of nucleic acids), and then carrying out thesynthesis of a main chain by random sequential condensation, in whichnucleic acids are not determined, and a polymerase extension reactionusing primers on consensus sequences.

In the second embodiment of the present invention, preferably, y₁ and y₂respectively comprise nick-generating enzyme recognition sites, and morepreferably, the nick-generating enzyme is Nb.BtsI. Further, y₁ and y₂may preferably comprise restriction enzyme sites.

Regarding a Third Embodiment of the Present Invention

The third embodiment of the present invention provides a preferredadapter (B) used in the method of the above described second embodimentof the present invention, namely, an adapter for producing a circularDNA, consisting of two DNA strands complementary to each other havingthe following structure y₁-Y-y₂-X:

whereinX represents a double-stranded DNA being a site for a restriction enzymerecognizing a palindromic sequence;y₁ and y₂ represent double-stranded DNAs respectively comprisingidentical nick-generating enzyme recognition sites or restriction enzymesites being reversely oriented to each other;Y represents a double-stranded DNA sequence having a unique sequence foreach DNA molecule to be circularized,n represents an integer of 1 to 40, N₁ to N_(n), which may be the sameor different, each represent a deoxyribonucleotide selected from thegroup consisting of dAMP, dCMP, dGMP and dTMP, and N′₁ to N′_(n),represent the following deoxyribonucleotides respectively correspondingto the N₁ to N_(n),

TABLE 4 N_(k) N′_(k) dAMP dTMP dCMP dGMP dGMP dCMP dTMP dAMPwherein k represents an integer from 1 to n.

In the above structure, n is 1 to 40, preferably 4 to 15, and morepreferably 5 to 10. However, even if n is more than 40, the presentinvention can be carried out.

As with the above described second embodiment, in the third embodimentof the present invention, preferably, y₁ and y₂ respectively comprisenick-generating enzyme recognition sites, and more preferably, thenick-generating enzyme is Nb.BtsI or Nt.BsqQI. Further, y₁ and y₂ maypreferably comprise restriction enzyme sites.

Regarding a Fourth Embodiment of the Present Invention

The fourth embodiment of the present invention provides a kit forproducing a circular DNA, comprising the preferred adapter (A) andadapter (B) used in the method of the above described second embodimentof the present invention. That is to say, the fourth embodiment of thepresent invention provides a kit for producing a circular DNA,comprising an adapter (A) consisting of double-stranded DNA comprising arestriction enzyme site identical to the restriction enzyme siterepresented by X according to the above described third embodiment, andan adapter (B) according to the above described third embodiment.

The kit for producing a circular DNA of the present invention comprisesan adapter (A) and an adapter (B). These adapters are allowed to bind toboth ends of a DNA molecule of interest, and thereafter, the two-stepcircularization method according to the first or second embodiment ofthe present invention is carried out, so as to provide a circularizedDNA that comprises a circularized DNA formed only from a single-moleculeDNA without comprising a circularized DNA formed from amultiple-molecule DNA.

Regarding a Fifth Embodiment of the Present Invention

The fifth embodiment of the present invention provides a method forproducing a cDNA library, comprising using the two-step circularizationmethod according to the first or second embodiment of the presentinvention. By applying the method according to the first or secondembodiment of the present invention to a library consisting of linearcDNAs, a cDNA library comprising a circularized DNA consisting only of asingle-molecule DNA can be produced.

Regarding a Sixth Embodiment of the Present Invention

The sixth embodiment of the present invention provides a method foridentifying a gene by subjecting a circular DNA molecule obtained byusing the method according to the first or second embodiment of thepresent invention to a mate-pair method, the method comprising thefollowing steps:

1) a step of reading nucleotide sequences, each consisting of 15 to 600nucleotides, flanking to both sides of an adapter (B) in a circular DNAmolecule obtained by using the method according to the first or secondembodiment of the present invention; and2) a step of identifying a gene contained in the circular DNA moleculeby comparing the read nucleotide sequences with the sequences of bothends of a known gene.

Step 1) in the method of the sixth embodiment of the present inventionis a step of reading nucleotide sequences, each consisting of 15 to 600nucleotides, flanking to both sides of the adapter (B) in the circularDNA molecule obtained using the method according to the first or secondembodiment of the present invention. The nucleotide sequence to be readconsists of preferably 15 to 100 nucleotides, and more preferably 25 to35 nucleotides. However, the method of the present invention can also becarried out by reading a nucleotide sequence consisting of 600 or morenucleotides. The nucleotide sequence can be read by a method well knownto a person skilled in the art, using a sequencer.

Step 2) in the method of the sixth embodiment of the present inventionis a step of identifying a gene contained in the circular DNA moleculeby comparing the read nucleotide sequences with the sequences of bothends of a known gene. If the read nucleotide sequences, which correspondto the sequences on both ends of a DNA molecule of interest, can beconfirmed to be identical to the sequences of both ends of a known gene,then the DNA molecule of interest is identified to be the known gene.

Regarding a Seventh Embodiment of the Present Invention

The seventh embodiment of the present invention provides a method fordetecting a fusion gene by subjecting a circular DNA molecule obtainedby using the method according to the first or second embodiment of thepresent invention to a mate-pair method, the method comprising thefollowing steps:

1) a step of reading nucleotide sequences, each consisting of 15 to 600nucleotides, flanking to both sides of an adapter (B) in a circular DNAmolecule obtained by using the method according to the first or secondembodiment of the present invention; and2) a step of comparing the read nucleotide sequences with the sequencesof both ends of a known gene, wherein if the genes of both endscorrespond to known different genes, the gene comprised in the circularDNA molecule is identified to be a fusion gene.

Step 1) in the method of the seventh embodiment of the present inventionis a step of reading nucleotide sequences, each consisting of 15 to 600nucleotides, flanking to both sides of an adapter (B) in a circular DNAmolecule obtained by using the method according to the first or secondembodiment of the present invention. The nucleotide sequence to be readconsists of preferably 15 to 100 nucleotides, and more preferably 25 to35 nucleotides. However, the method of the present invention can also becarried out by reading a nucleotide sequence consisting of 600 or morenucleotides. The nucleotide sequence can be read by a method well knownto a person skilled in the art, using a sequencer.

Step 2) in the method of the seventh embodiment of the present inventionis a step of comparing the read nucleotide sequences with the sequencesof both ends of a known gene. Herein, if the genes of both endscorrespond to known different genes, the gene comprised in the circularDNA molecule is identified to be a fusion gene. That is to say, if aportion, which corresponds to one end of the read nucleotide sequencescorresponding to the sequences on both ends of a DNA molecule ofinterest, is identical to one end of a known gene, and if a portioncorresponding to the other end is identical to one end of another knowngene, the DNA molecule of interest is identified to be a fusion geneconsisting of two known genes.

As an example, when the sequences of both sides flanking to both sidesof the adapter (B) correspond to both ends of a known fusion gene, theDNA of interest is detected to be the known fusion gene. When therelationship between the expression of the known fusion gene and adisease has been known, it is possible to detect a disease characterizedby the expression of the fusion gene detected by the aforementionedmethod, using the fusion gene as a marker.

As another example, when the sequences flanking to both sides of theadapter (B) correspond to the terminal sequences of different genes anddo not correspond to both ends of a known fusion gene, the genecontained in the circular DNA molecule is identified to be a novelfusion gene. The novel fusion gene detected by the aforementioned methodcan be used in drug discovery screening.

EXAMPLES

Specific methods of allowing adapters to bind to both ends of a DNAmolecule of interest (the left and right ends of a DNA molecule ofinterest) will be described in the following examples.

Example 1 Application of the Method of the Present Invention to theDiscovery of a Fusion Gene, in which cDNA Synthesized from mRNA is Used

When a cDNA library is synthesized from mRNA, in the most common method(Clontech SMART cDNA method) among the currently used methods,complementary strand DNA is first synthesized with reversetranscriptase, using oligonucleotide (1) having a poly T sequencecomplementary to a poly A site at the 3′-terminus of mRNA, as shown inFIG. 7. After completion of the synthesis, a specific oligonucleotidesequence (2) is incorporated at the 5′-terminus of mRNA, as shown inFIG. 7. Subsequently, DNA is synthesized from an oligonucleotidecomplementary to this specific sequence, or a cDNA library is producedby PCR.

In Case the Adapters of the Present Invention are Incorporated into cDNAat the Same Time with the cDNA Synthesis

In this case, the adapter (B) has been introduced into theoligonucleotide (1) sequence having a poly T sequence complementary to apoly A site at the 3′-terminus of mRNA, and the adapter (A) has beenadded to the oligonucleotide (2) sequence. Thus, when a cDNA library issynthesized, it automatically becomes the basic structure of the presentinvention. It is not necessary to bind new sequences to the right andleft ends, and the routine can directly proceed to the next step. Bythis method, when the cDNA library is further amplified by PCR, if it isdirectly amplified, a specific N nucleotide sequence must be amplified.Hence, since the diversity of the N nucleotide is maintained,amplification is carried out using, for example, a 5′ phosphategroup-added primer as an upstream primer, and a primer containing the Nnucleotide as a downstream primer, and after completion of the PCR, astrand into which the phosphate group primer has been incorporated isdigested with λ exonuclease. Thereafter, primer-extension is carried outagain from the upstream primer, so as to produce a cDNA library in whichthe diversity of the N nucleotide is maintained. However, in the case ofthe adapter binding method, this process is not necessary. In order tointroduce the adapter of the present invention after modification, suchas fragmentation of the library, the subsequent adapter binding methodis carried out.

In Case the Adapter of the Present Invention is Incorporated into cDNAafter Ordinary cDNA Synthesis

When a cDNA library is produced, it is possible that a restrictionenzyme sequence has been introduced into each of the 3′-terminus and5′-terminus thereof, as shown in FIG. 7(3). As a restriction enzyme usedin this case, either an ordinary palindromic sequence, or anon-palindromic sequence for distinguishing the 3′-terminus and5′-terminus, can be used. Moreover, in order to ensure adapter binding,the adapter can also be bound to an end in the shape of blunt end withan A-protruding nucleotide. In order to introduce the adapter of thepresent invention after modification of the library, this method isdesirable.

Example 2 Application of the Method of the Present Invention to GenomicAnalysis by Mate-Pair Method

When genome is targeted, the situation is different from the case of acDNA library. Differing from cDNA, a genomic fragment cannot distinguishthe left and right sides of a DNA fragment. Accordingly, as shown inFIG. 8, only the product (a) in FIG. 8, namely, a product prepared bybinding the adapter on the left end to the adapter on the right end sothat the two adapters are appropriately ligated, can be selected by aPCR method or a biotinylation method, or by inserting non-palindromicsequences as a restriction enzyme X sequence into both ends, or a methodof establishing a plurality of restriction enzyme sites at the samesite, using restriction enzyme (BstXI, etc.) containing an N region,etc.

Regarding Adapters (A) and (B) Added to Both Ends of a DNA Molecule

Basically, the following method is adopted to add adapters (A) and (B)to both ends of a DNA molecule. At first, to a group of a plurality ofDNAs, the concentration of which has been almost determined depending onthe number of DNA molecules of interest, an adapter (A) is first allowedto bind. In this case, the amount of the adapter (A) added is set to besmaller than the amount of the target DNA molecules. However,stoichiometrically, the number of the adapters (A) may be the same asthe number of the target DNA molecules, or one nucleotide protrusion maybe enzymatically produced on a DNA fragment and a complementary adaptermay be allowed to bind thereto. Especially, in this case, theconcentration of the adapter may be excessive. Thereby, the adapter (A)is allowed to bind to the end of the DNA molecule. Subsequently, anadapter (B) is allowed to bind.

In the present invention, as shown in (a) of FIG. 8, it is necessary toproduce a molecule in which the adapter (A) binds to one end thereof andthe adapter (B) binds to the other end. Herein, as shown in (b) or (c)of FIG. 8, it is necessary to eliminate a DNA molecule to which only theadapter (A) binds ((b) of FIG. 8) and a DNA molecule to which only theadapter (C) binds ((c) of FIG. 8). The methods therefor will bedescribed below.

In Case of Using a Non-Palindromic Sequence to a First RestrictionEnzyme X Site

In the case of a molecule of type (a) of FIG. 8, a firstcircularization, nick cleavage, and a second circularization are carriedout without problems, and as a result, circularized DNA of interest isproduced without problems.

On the other hand, in the case of a molecule of type (b) of FIG. 8, thefirst circularization is impossible for a single molecule and thus, itis eliminated. However, circularization with a plurality of DNAs can beformed (for example, the binding of two molecules, namely, the bindingof the molecule of type (b) to the molecule of type (c)). Such aplurality of DNA molecule must bind to the molecule of type (a) or type(c) as a binding partner. Accordingly, both sides must bind to specificsequences having N sequences. Thus, after completion of the nickcleavage, the second circularization cannot be formed and is eliminated.

Moreover, in the case of a molecule of type (c) of FIG. 8, the firstcircularization is impossible for a single molecule and is eliminated.However, circularization with a plurality of DNAs can be formed (forexample, the binding of two molecules, namely, the binding of themolecule of type (b) to the molecule of type (c)). Since this DNAfragment originally has different N sequences on both ends, aftercompletion of the nick cleavage, the second circularization cannot beformed and is eliminated.

In Case of Using a Palindromic Sequence to a First Restriction Enzyme XSite

In the case of a molecule of type (a) of FIG. 8, a firstcircularization, nick cleavage, and a second circularization are carriedout without problems, and as a result, circularized DNA of interest isproduced without problems.

On the other hand, in the case of a molecule of type (b) of FIG. 8, thefirst circularization is possible. However, since this molecule does nothave a nick sequence, nick cleavage is not carried out, and it remainsas circularized DNA and cannot be eliminated. As an example of a methodof preventing this, there is a BstXI method as described below.

Moreover, in the case of a molecule of type (c) of FIG. 8, the firstcircularization and nick cleavage are possible. However, since thismolecule originally has different N sequences on both ends, aftercompletion of the nick cleavage, the second circularization cannot beformed and is eliminated.

Regarding BstXI method

In a case in which adapters (A) are added to both ends, both ends areligated at a first stage. However, since cleavage does not take place ata second stage, it remains as circular DNA. As a method of solving thisproblem, BstXI is added to the outside of the adapter (A) comprising therestriction enzyme site, for example. Specifically, into the followingrestriction enzyme BstXI site, an EcoRI site is incorporated, so thatCCANNNNNNTGG is converted to CCAGAATTCTGG, for example.

That is to say, in this method, the restriction enzyme site of each ofthe adapter (A), and the adapter (A) contained in the adapter (B), isset to be an EcoRI site, for example. Then, only in the case of theadapter (A), the sequence outside the EcoRI site is set to be a BstXIrecognition sequence, for example. That is, the sequence of the adapter(A) is set to be CCAGAATTCTGG. On the other hand, the adapter (A)contained in the adapter (B) comprises an EcoRI site but does notcomprise a BstXI recognition sequence, so that it cannot be cleaved withBstXI.

As a result, if the two adapters (A) bind to both ends and are thenassociated with each other, circularization can be opened by cleavingwith BstXI and can be then eliminated. Of course, if the adapter (A)binds to the adapter (B), since the adapter (B) does not have a BstXIrecognition sequence, it cannot be cleaved with BstXI.

According to the above described method, the method of the presentinvention can be applied even to a genomic DNA fragment that cannotdistinguish between upstream and downstream, differing from cDNA, sothat single-molecule circular DNA can be produced, and so that theaccuracy of mate-pair analysis can be significantly improved.

Example 3 Regarding the Possibility of Rebinding of Circular DNA Genesin a Plurality of Circular DNA Molecules

Such possibility can be fully ignored, stochastically. The grounds willbe described below.

In order to stochastically assume the possibility that separated genesare re-associated, such possibility is estimated in the following threecases: (1) a case in which the possibility is assumed from the volume ofa DNA molecule and the amount (volume) of a reaction solution; (2) acase in which the possibility is assumed from the number of molecules inthe reaction solution (in the two above cases, it is assumed that thenumber of N nucleotides in the Y portion is sufficiently large and thatit has high specificity); and further, (3) a case in which the number ofN nucleotides is determined but it is not sufficiently large.

(1) The case in which the possibility is assumed from the volume of aDNA molecule and the amount (volume) of a reaction solution

First, a single DNA molecule is presumed to be a sphere and its volumeis estimated. Then, the possibility that molecules that have once beenseparated in a reaction system are associated as spheres in a solutionis estimated.

The length of one nucleotide: 0.34 nm (0.34×10e-9 m=3.4×10e-8 mm),

the length of a 3-kbp (3000 bp) plasmid: 1×10.0e-4 mm,

the volume of a DNA molecule assumed to be a sphere:4/3×3.14×(1×10e-4)×(1×10e-4)×(1×10e-4) mm3=4×10e-12 mm3, and

the number of DNA molecules as spheres in 100 ul if the volume of asingle molecule is presumed to be 4×10e-12 mm3: 100 mm3/4×10e-12mm3=2.5×10e13.

Accordingly, if the reaction system is homogeneous, the possibility thata sphere is associated with another equivalent sphere complementarythereto is extremely small ((1/(2.5×10e13))=4×10e-14).

(2) The case in which the possibility is assumed from the number ofmolecules in the reaction solution

On the other hand, the number of molecules is calculated as follows.

The molecular weight of a 3-kbp plasmid: 625×3000=1.8×10e6,

the mass of 1 mole of plasmid: 1 mol/L=1.8×10e6 g/1 L=1.8×10e12 ug/1 L,

the molar number of 3 ug of plasmid: 1 (mol/L)×3 ug/(1.8×10e12)ug=3/1.8×10e-12 mol/L=1.6×10e-12 mol/L=1.6×10e-9 mol/ml, and

the number of molecules of 3 ug of plasmid dissolved in 1 ml of solutionif the Avogadro's number is presumed to be 6×10e23: 1.6×10e-9mol/ml×6×10e23=1.6×6×10e14=1×10e15.

Accordingly, if 3 ug of plasmid is present in 100 ul of reaction system,since 100 ul=10e-1 mm3, the number of molecules is 1×10e14.

The possibility that a molecule in the solution with the aforementionednumber of molecules is associated with another equivalent moleculecomplementary thereto is extremely small (1/1×10e14).

(3) As described above, it is assumed that the specificity of the numberof N nucleotides in the Y portion would rather contribute to rebindingof separated genes.

When the number of N nucleotides is “5,” the possibility that twoidentical genes separated are rebound via identical gene fragments is:((¼)×(¼)×(¼)×(¼)×(¼))×((¼)×(¼)×(¼)×(¼)×(¼))=1×10e-6.

Accordingly, from these estimations, it can be concluded that thepossibility that, after a plurality of DNAs have formed a ring and theyhave been then separated, identical DNA molecules rebind to each other,is extremely low, and thus, it can be fully ignored.

Example 4 Production of Circularized DNA from Single-Molecule DNA

The following experiment was carried out to verify that noise incircularization with a plurality of DNA molecules is reduced byincreasing the efficiency of circularization with single-molecule DNAaccording to a two-step DNA binding (ligation) method.

Two types of plasmids, Plasmid A and Plasmid B, were prepared. Product(i) obtained by cleaving each plasmid with a restriction enzyme and thendirectly ligating the fragments was compared with product (ii) obtainedby ligating the adapter of the present invention according to thetwo-step ligation.

In the case of the product (ii) to which the method of the presentinvention was applied, any given nucleotides N (6 nucleotides) used asan adapter were subjected to random synthesis, and after ligation of theadapter, the adapter was cleaved at an NcoI site present on the end ofthe adapter. Then, a mixture of these plasmids was circularized by afirst-step ligation, and using Nick enzyme, Nick was placed before andafter any given 6 nucleotides N, so that circularization was cleaved.Thereafter, ligation was carried out again to perform a second-stepcircularization.

It is predicted that, during this operation, the products (i) and (ii)in the mixed solution would comprise (1) circularized DNA formed with asingle molecule of Plasmid A, (2) circularized DNA formed with a singlemolecule of Plasmid B, (3) circularized DNA formed with a plurality ofmolecules of Plasmid A, (4) circularized DNA formed with a plurality ofmolecules of Plasmid B, and (5) circularized DNA formed with a pluralityof molecules comprising Plasmid A and Plasmid B.

According to the present invention, it was anticipated that theabove-mentioned circularized DNAs (3), (4), and (5) would besignificantly decreased in the product (ii), in comparison with theproduct (i).

Hence, the amount of the circularized DNA (5) in the mixed solutiongenerated from these products (i) and (ii) was evaluated as arepresentative example. Quantitative PCR was carried out using PCRprimers specific to Plasmid A and Plasmid B.

As a result, it was confirmed that the amount of the circularized DNAformed with a plurality of molecules such as (5) above, which wasconsidered as noise, could be reduced to approximately one-hundredth byapplying the method of the present invention (ii), in comparison withthe method (i).

INDUSTRIAL APPLICABILITY

Using the adapter and method of the present invention, the accuracy ofgene analysis is significantly improved. In addition, unconventionalhigh-accuracy mate-pair analysis becomes possible, and thus, the adapterand method of the present invention are provided as tools extremelyuseful for genomic analysis. In particular, by applying the method ofthe present invention to the production of a cDNA library, it is highlylikely to discover a novel fusion gene. That is to say, by reliablyachieving circularization of a single DNA molecule, it becomes possibleto develop novel diagnostic tool and/or method.

According to the present invention, a method for circularizing only asingle-molecule DNA, wherein circularization of a plurality of moleculesis substantially prevented with regard to circularization of DNA, isprovided, so that problems regarding contamination in gene analysis suchas mate-pair analysis can be solved, and so that high-accuracy analysisbecomes possible. Moreover, by applying the method of the presentinvention to detection and/or analysis of a fusion gene, high-accuracyanalysis of a fusion gene becomes possible, and a useful diagnostic toolcan be provided.

1-19. (canceled)
 20. An adapter for producing a circular DNA, theadapter consisting of two DNA strands complementary to each other havingthe following structure y₁-Y-y₂-X:

wherein X represents a double-stranded DNA being a restriction enzymerecognition site comprising a palindromic sequence; y₁ and y₂ representdouble-stranded DNAs respectively comprising identical nick-generatingenzyme recognition sites being reversely oriented to each other; Yrepresents a double-stranded DNA sequence having a unique sequence foreach DNA molecule to be circularized, n represents an integer of 4 to15, N₁ to N_(n), which may be the same or different, each represent adeoxyribonucleotide selected from the group consisting of dAMP, dCMP,dGMP and dTMP, and N′₁ to N′_(n) represent the followingdeoxyribonucleotides respectively corresponding to the N₁ to N_(n),TABLE 1 N_(k) N′_(k) dAMP dTMP dCMP dGMP dGMP dCMP dTMP dAMP

wherein k represents an integer from 1 to n.
 21. The adapter accordingto claim 20, wherein the nick-generating enzyme is Nb.BtsI or Nt.BsqQI.22. A kit for producing circular DNA, comprising an adapter consistingof a double-stranded DNA comprising a restriction enzyme recognitionsite identical to the restriction enzyme recognition site represented byX defined in claim 20, and the adapter according to claim 20.